1. Field of the Invention
The present invention relates to a sputtering target for fabricating a recording layer of an optical recording medium; a method of producing the sputtering target; a phase-change type optical recording medium comprising a recording material in a recording layer thereof, which recording material is capable of causing changes in the phase thereof by the application of a laser beam thereto, thereby recording, reproducing and overwriting information therein; and a method of fabricating the above-mentioned phase-change type optical recording medium by using the above-mentioned sputtering target.
2. Discussion of Background
There is conventionally known a phase-change type optical information recording medium which utilizes phase changes between a crystalline phase and an amorphous phase or between one crystalline phase and another crystalline phase as one of the optical recording media which are capable of recording, reproducing and erasing information by the application thereto of electromagnetic waves, such as a laser beam. This kind of phase-change type optical information recording medium enables the overwriting of information by the application of a single laser beam thereto, although such overwriting is difficult to conduct by the application of a single laser beam in magneto-optical memory using a magneto-optical recording medium. An optical system of a drive unit for the phase-change type optical information recording medium can be designed simpler than that for a magneto-optical recording medium, so that recently research and development of this kind of recording medium has been actively conducted.
As disclosed in U.S. Pat. No. 3,530,441, the so-called chalcogen-based alloys, such as Gexe2x80x94Te, Gexe2x80x94Texe2x80x94Se, Gexe2x80x94Texe2x80x94S, Gexe2x80x94Sexe2x80x94S, Gexe2x80x94Sexe2x80x94Sb, Gexe2x80x94Asxe2x80x94Se, Inxe2x80x94Te, Sexe2x80x94Te and Sexe2x80x94As, are conventionally used as recording materials for the phase-change type optical recording medium. In addition, it is proposed to add an element of Au to the above-mentioned Gexe2x80x94Te based alloy to improve the stability and to increase the rate of crystallization of the recording material as disclosed in Japanese Laid-Open Patent Application 61-219692. Furthermore, the addition of Sn and Au to the Gexe2x80x94Te based alloy, and the addition of Pd to the same are respectively proposed in Japanese Laid-Open Patent Applications 61-270190 and 62-19490 for the same purposes as mentioned above. Furthermore, recording materials comprising a mixture of Ge, Te, Se and Sb, and a mixture of Ge, Te and Sb are respectively disclosed in Japanese Laid-Open Patent Applications 62-73438 and 63-228433, each of which has a specific composition in terms of atomic percent of each constituent element of the recording material for the improvement of the recording and erasing repetition properties of the obtained recording medium.
However, none of the above-mentioned conventional phase-change type optical recording media satisfies all the requirements for the phase-change type rewritable optical recording medium. In particular, most important points to be improved in the conventional phase-change type optical recording media are to improve the recording sensitivity and erasing sensitivity, and to prevent the decrease of the erasability in the course of the overwriting operation, which is caused by the portions remaining unerased.
For the above-mentioned improvements, a mixed-phase type recording material comprising an constituent elements Ag, In, Sb and Te is proposed, as disclosed in, Japanese Laid-Open Patent Applications 3-240590, 4-78031, 4-232779 and 5-345478. Such a mixed-phase type recording material can show the recording and erasing sensitivities to the light with a peak power of 12 mW or less, and excellent erasabillty, so that it is effective when used in a recording layer for mark edge recording. However, conventional optical recording media employing the Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te based recording materials are still insufficient with respect to the shelf stability and the repetition reliability.
Accordingly, a first object of the present invention is to provide a sputtering target for fabricating a recording layer of an optical recording medium with excellent C/N ratio, high erasability and sensitivities, minimum jitter, and sufficient shelf stability and repetition reliability.
A second object of the present invention is to provide a method of producing the above-mentioned sputtering target.
A third object of the present invention is to provide an optical recording medium with excellent C/N ratio, high erasability and sensitivities, minimum jitter, and sufficient shelf stability and repetition reliability.
A fourth object of the present invention is to provide a method of producing the above-mentioned optical recording medium.
The first object of the present invention can be achieved by a sputtering target for fabricating a recording layer of an optical recording medium capable of recording and erasing information by utilizing changes in the phase of a recording material in the recording layer, the sputtering target comprising a compound or mixture comprising as constituent elements Ag, In, Te and Sb with the respective atom c percent (atom. %) of xcex1, xcex2,xcex3 and xcex4 thereof being in the relationship of 2xe2x89xa6xcex1xe2x89xa630, 3xe2x89xa6xcex2xe2x89xa630, 10xe2x89xa6xcex3xe2x89xa650, 15xe2x89xa6xcex4xe2x89xa683, and xcex1+xcex2+xcex3+xcex4=100.
It is preferable that the compound or mixture for use in the above-mentioned sputtering target comprise Sb, and AgInTe2 with a staichiometric composition and/or a nearly stoichiometric composition having a chalcopyrite structure, and it is preferable that the AgInTe2 form crystallites with a particle size of 450 xc3x85 or less.
The second object of the present invention can be achieved by a method of producing a target for sputtering, comprising the steps of mixing Ag, In and Te elements to prepare a mixture of Ag, In and Te elements, fusing the mixture of Ag, In and Te elements at 600xc2x0 C. or more to prepare a fused mixture, rapidly cooling the fused mixture to prepare a solid lump, pulverizing the solid lump to prepare finely-divided particle, mixing the finely-divided particles with Sb to prepare a mixture of the finely-divided particles and Sb, and sintering the mixture of the finely-divided particles and Sb.
Alternatively, the second object of the present invention can also be achieved by a method of producing a target for sputtering, comprising the steps of mixing Ag, In, Te and Sb elements to prepare a mixture of Ag, In, Te and Sb elements, fusing the mixture of Ag, In, Te and Sb elements at 600xc2x0 C. or more to prepare a fused mixture, and rapidly cooling the fused mixture to prepare a solid lump, pulverizing the solid lump to prepare finely-divided particles, and sintering the finely-divided particles.
In either case, it is preferable that the method of producing the sputtering target further comprise the step of carrying out the heat treatment at a temperature not higher than the melting point of the mixture prior to the sintering step.
The third object of the present invention can be achieved by an optical recording medium comprising a recording layer capable of recording and erasing information by utilizing changes in the phase of a recording material in the recording layer, the recording layer comprising as constituent elements Ag, In, Te and Sb with the respective atomic percent of xcex1, xcex2, xcex3 and xcex4 thereof being in the relationship of 0 less than xcex1xe2x89xa630, 0 less than xcex2xe2x89xa630, 10xe2x89xa6xcex3xe2x89xa650, 10xe2x89xa6xcex4xe2x89xa680, and xcex1+xcex2+xcex3+xcex4=100.
The third object can also be achieved by an optical recording medium comprising a recording layer capable of recording and erasing information by utilizing changes in the phase of a recording material in the recording layer, the recording layer comprising an constituent elements Ag, In, Te, Sb, and nitrogen and/or oxygen atom with the respective atomic percent of xcex1, xcex2, xcex3, xcex4 and xcex5 thereof being in the relationship of 0 less than xcex1xe2x89xa630, 0 less than xcex2xe2x89xa630, 9xe2x89xa6xcex3xe2x89xa650, 9xe2x89xa6xcex4xe2x89xa680, 0 less than xcex5xe2x89xa65, and xcex1+xcex2+xcex3+xcex4xcex5=100, wherein xcex5 is the total atomic percent of the nitrogen and/or oxygen atom.
In any case, the atomic percent of xcex1 and xcex3 in the composition of the above-mentioned recording layer may be in the relationship of xcex1xe2x88x92xcex3/2xe2x89xa6xe2x88x928.
From the aspect of the structure of the optical recording medium, it is preferable that the above-mentioned optical recording medium comprise a polycarbonate substrate, a first protective layer, the previously mentioned recording layer, a second protective layer, a reflective heat dissipation layer and an ultraviolet curing resin layer, which are overlaid in this order.
Furthermore, in the above-mentioned optical recording medium, it is preferable that the first and second protective layers employ the same material, with the total of the thermal stress and the true stress of each protective layer being at least 150 MPa. Further, it is preferable that the yield stress of the recording layer be at least 200 MPa.
In addition, the previously mentioned recording layer may comprise a nitride and/or oxide comprising at least one of the constituent elements Ag, In, Te and Sb; or a nitrogen atom, and may further comprise an element which is capable of forming an alloy or compound in combination with any of the constituent elements Ag, In, Te and Sb.
The fourth object of the present invention can be achieved by a method of producing an optical recording medium comprising a recording layer, comprising the step of forming the recording layer in a sputter chamber by sputtering by use of a sputtering target which comprises a compound or mixture comprising as constituent elements Ag, In, Te, and Sb with the respective atomic percent of xcex1, xcex2, xcex3 and xcex4 thereof being in the relationship of 2xe2x89xa6xcex1xe2x89xa630, 3xe2x89xa6xcex2xe2x89xa630, 10xe2x89xa6xcex3xe2x89xa650, 15xe2x89xa6xcex4xe2x89xa683, and xcex1+xcex2+xcex3+xcex4=100, in a gas comprising argon gas and nitrogen gas with a concentration of 0 to 15 mol %.
In the above-mentioned method of producing the recording medium, it is preferable that the back pressure be set in the range of 3xc3x9710xe2x88x927 to 5xc3x9710xe2x88x926 Torr prior to the sputtering process.
Further, it is preferable that a gas containing nitrogen gas with a concentration higher than the concentration of nitrogen in the gas employed during the sputtering process be introduced into the sputter chamber after sputtering process.